Method of providing electric power with thermal protection

ABSTRACT

An electric machine including a magnetic component, forming part of its rotor or stator that loses its magnetic characteristics above a certain chosen temperature is disclosed. This magnetic material forms part of a magnetic circuit that guides flux about the stator. As a result, any magnetic flux emanating with the rotor stops circulating about the stator above this temperature, and the machine stops acting as generator. The component is thermally coupled to windings carrying current from the machine&#39;s stator. The material forming the component is selected so that the chosen temperature is lower than the temperature at which the machine would be thermally damaged. This, in turn, limits the operating temperature of the windings, and thus prevents overheating of the machine during operation, typically caused by a fault. Preferably this magnetic material is formed from a ferrite material, such as a Manganese Zinc ferrite material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/949,855,which is a divisional application of Ser. No. 09/467,761, now U.S. Pat.No. 6,313,560.

FIELD OF THE INVENTION

The present invention relates to electric machines, and moreparticularly to electric generators that are thermally protected fromdamage resulting from high currents in such machines.

BACKGROUND OF THE INVENTION

Permanent magnet electric motors and generators are well known andunderstood. Typically, such permanent magnet machines include a rotorformed, at least in part, from a magnetic material such asSamarium-Cobalt. Electric windings on a stator about the rotor are usedto carry current that either generates a magnetic field or is the resultof a magnetic field about the rotor. As a motor, current through thewindings induces the rotating magnetic field, which in turn applies atorque to the magnetic portion of the rotor causing it to act as motor.Similarly, as a generator, torque applied to the rotor, results in arotating magnetic field that induces a current in the windings.

Such electric machines provide significant benefits over synchronousmachines, squirrel cage motors and other types of electric machines.Significantly, permanent magnet machines do not require brushes; arerelatively light; use conventional and developed electronics to generateany required rotating magnetic field; and can act as:both motors andgenerators.

In view of these benefits, such machines appear well suited for aircraftapplications. Particularly, such machines would appear to lendthemselves for use as starters and generators within a turbine engine.

Conveniently, such machines can be connected directly to the engineshaft. When required, generated electricity can be rectified andfiltered using conventional lightweight electronics. When DC currentsare required, as in traditional aircraft applications, the speed ofrotation and frequency of generator output does not need be controlled.Heavy gearing is therefore not required. Operating as motors, suchmachines can act as starters.

Disadvantageously, however, machines coupled to such engines canpotentially generate extreme power limited only by the power of theturbine engine driving the rotor of the machine. Unabated, generation ofsuch electric power can result in extreme heat, particularly in thestator windings, that may cause the motor to melt and potentially burn.This is clearly undesirable. Obviously, current provided by the machineto interconnected electrical equipment may be limited by fusing theinterconnected equipment or even the electronics used to rectify orregulate AC currents. However, such fusing will not react to shortcircuits internal to the machine. While unlikely, such short circuitsmight, for example, occur in the stator windings. Should this happen, apermanent magnet machine will invariably overload and overheat causingdamage to the machine, and perhaps even to the associated engine. In theextreme case, this may cause the main engine to fail as a result of thehigh temperature of the engine shaft coupled to the motor. Similarproblems may be manifested in other types of electric machines.

Accordingly, an improved electric machine that is thermally protected isdesirable.

SUMMARY OF THE INVENTION

In accordance with the present invention, an electric machine includes aferrite portion, forming part of its rotor or stator that loses itsmagnetic characteristics above a certain chosen temperature. As aresult, any magnetic flux circulating between the rotor and stator issignificantly reduced above this temperature, and the machine stopsacting as generator. The component is thermally coupled to windingscarrying current from the machine's stator. The material forming thecomponent is selected so that the certain temperature is lower than thetemperature at which the machine would be thermally damaged. This, inturn, limits the operating temperature of the windings, and thuspreventing overheating of the machine during operation.

In accordance with an aspect of the present invention an electricmachine includes a permanent magnet motor and a stator mounted about therotor, at least partially forming a magnetic circuit guiding a magneticflux emanating from the permanent magnet. At least one winding extendsabout the stator for picking up a current induced by the magnetic flux.At least a portion of the magnetic circuit is thermally coupled to thewinding and is formed from magnetic material having a Curie temperaturebelow a temperature at which the machine is damaged. This limits themagnetic flux about the magnetic circuit above the Curie temperature,and thus limits the operating temperature of the windings, and preventsoverheating of the machine during operation.

In accordance with another aspect of the invention, an electricgenerator includes a rotor assembly including a permanent magnet; and astator formed of a ferrite material mounted about the rotor, at leastpartially forming a magnetic circuit guiding a magnetic field emanatingfrom the permanent magnet. At least one winding extends about the statorfor picking up a current induced by the magnetic field. Preferably, theferrite material is a Manganese-Zinc ferrite material.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art, upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In figures which illustrate, by way of example only, preferredembodiments of the invention,

FIG. 1 is an exploded view of a permanent magnet machine, exemplary ofan embodiment of the present invention;

FIG. 2 is a cross sectional view of the machine of FIG. 1;

FIG. 3 is an exploded view of a partial stator assembly that may formpart of the machine of FIG. 1;

FIG. 4 is a side perspective view of an exemplary stator assemblyforming part of the machine of FIG. 1;

FIG. 5 is a back end view of FIG. 4;

FIG. 6 is a front end view of FIG. 4;

FIG. 7 schematically illustrates the flow of current about the statorassembly of FIG. 4; and

FIG. 8 is a top view of a portion of a further stator that may be usedwith a machine exemplary of a further embodiment of the presentinvention; and

FIG. 9 shows a gas turbine engine incorporating the present invention,with a portion of the engine broken away to reveal a cross-sectionthereof.

DETAILED DESCRIPTION

FIGS. 1 and 2 illustrate a permanent magnet electric machine 10,exemplary of an embodiment of the present invention. As illustrated,electric machine 10 includes a stator assembly 12 and rotor assembly 14,preferably mounted within a housing 16. Rotor assembly 14 is mounted forfree rotation about its central axis within housing 16 by bearings 20and 22.

Housing 16 includes an outer cylindrical shell 24, and generally discshaped front and rear end plates 26 and 28. End plates 26 and 28 arefixed to shell 24, and thereby retain stator assembly 12, rotor assembly14, and bearings 20 and 22 within housing 16. Annular walls 30 and 32extend inwardly from the interior of end plates 26 and 28 and retainbearings 20 and 22 at defined axial positions within housing 16, aboutrotor assembly 14. A further retaining washer 23 assists to retainbearings 20 and 22. Housing 16 is preferably formed of high-gradestainless steel.

Example rotor assembly 14 includes a generally cylindrical core section38. Two smaller diameter cylindrical shafts 34 and 36 extend axiallyoutward from core section 38, toward the front and rear of housing 16,respectively. Spacing ledges 42, 44 and 46, 48 separate shafts 34 and36, respectively, from core section 38. Ledges 42 and 46 abut withbearings 20 and 22. A further smaller diameter concentric drive shaft 40extends axially outward from shaft 34 and the front of housing 16. Aswill be appreciated, core section 38; shafts 34, 36 and 40 arepreferably machined from a single piece of relatively low strengthmagnetic steel, such as maraging steel. A thin shell 18 formed ofnon-magnetic material, such as a Nickle alloy, at least partiallyencapsulates core section 38 and contains the relatively low strengthmagnetic steel. Shell 18 is preferably formed of AMS 5662 or AMS 5663Nickel Alloy and may be shrink fitted to the core portion 38 and thenground to achieve a desired overall thickness of shell 18.

Stator assembly 12 is further illustrated in FIGS. 3-7. As illustrated,stator assembly 12 includes a magnetic circuit defined by an exemplaryhollow cylinder 50. Cylinder 50 includes a plurality of lengthwiseextending, evenly spaced slots 52 a, 52 b and 52 c (collectively slots52) extending on its interior. In the preferred embodiment, a total ofeighteen such slots extend along the cylinder's length. Conveniently,the eighteen slots 52 a, 52 b and 52 c may be grouped into three groups,with all slots 52 a belonging to one group, all slots 52 b and 52 c toanother. Each third slot belongs in one of the groups. As bestillustrated in FIG. 3, a set of six rectangular conductors 54 a that arecomplementary in shape to slots 52 a, occupy the entire length of theseslots. Each of these conductors is formed of a material such as copper,and is insulated by a thin plastic coating. Each of conductors 54 a isidentical in length, and extends slightly beyond the ends of cylinder50. Adjacent conductors within the group of conductors 54 a areinterconnected by arced conductors 56 a extending radially about thecentral axis of cylinder 50, and exterior to cylinder 50. Alternatingpairs of conductors 54 a are connected at opposite ends of cylinder 50.Thus, two arced conductors 56 a are at one end of cylinder 50 and threeare at the opposite. Conductors 54 a and 56 a thus form an electriccircuit (referred to as circuit 58 a) traversing the length of cylinder50 six times, at intervals spaced sixty degrees about a central axis ofcylinder 50. Diametrically opposed rectangular conductors (ie. spaced byone-hundred and eighty degrees) have currents running in oppositedirection along the length of cylinder 50 and thus form current loops orwindings about the central axis of machine 10. As illustrated in FIGS.4-6, conductors 54 b, 56 b and 54 c, 56 c are similarly arranged tooccupy the remaining slots 52 b and 52 c, and thus form circuits 58 band 58 c. Resulting circuits 58 a, 58 b and 58 c (collectively circuits58) thus form nine current loops or windings about central axis ofmachine 10. As illustrated in FIG. 6, conductors 54 b and 54 c have thesame length as conductors 54 a and are arranged at axial positions sothat conductors 54 a, 54 b, 54 c (collectively conductors 54) and 56 a,56 b and 56 c (collectively conductors 56) are not in contact with eachother. Moreover, these conductors are preferably insulated so that theyare not electrically connected with cylinder 50, and are thermallycoupled to cylinder 50. The conductors may be coupled to cylinder 50 byway of a known thermal conductive varnish or epoxy. Cylinder 50 andconductors 54 may be encapsulated using this varnish or epoxy. Contactpoints for each circuit 58 a, 58 b and 58 c extend from the rear end ofcylinder 50, as illustrated in FIG. 5. Current flow in circuits 58 a, 58b and 58 c as viewed at the rear of machine 10, resulting from apotential difference across the contact points is schematicallyillustrated in FIG. 7.

As illustrated, stator assembly 12 and cylinder 50 are coaxial with coresection 38. A small air gap separates core section 38 from cylinder 50.

A conventional three phase circuit (not shown) may be used to drivecircuits 58 a, 58 b and 58 c to cause machine 10 to act as a motor.Specifically, driving circuits 58 results in a rotating magnetic fieldgenerated by the nine windings or current loops, travellingcircumferentially within cylinder 50. This field is guided by cylinder50 acting as part of a magnetic circuit about the center axis of thiscylinder 50, and in turn the core section 38 of rotor assembly 14. Aswill be appreciated by those of ordinary skill in the art, the rotatingmagnetic field exerts a torque on the magnetic portion of rotor assembly14, causing it to rotate.

Cylinder 50 is preferably formed of a ferrite material. As is understoodby those of ordinary skill in the art, ferrite materials exhibitmagnetic properties and have high relative permeability resulting in lowmagnetic reluctance, allowing such materials to guide magnetic flux.Perrites typically have cubic crystalline structure with the chemicalformula MO.Fe₂O₃, where MO is typically a combination of two or moredivalent metals, such as zinc, nickel, manganese or copper. Ferrites aretypically classified as “hard” or “soft”. “Soft” ferrite materials onlyexhibit significant magnetic characteristics in the presence of amagnetic field, while “hard” ferrite materials tend to permanentlyretain their magnetic characteristics. As is further, understood, thenature of most magnetic materials is typically temperature dependent.Most magnetic materials lose their magnetic properties above a criticaltemperature, referred to as the Curie temperature of the material. Formany materials, and for most ferrites, once the temperature of thematerial drops below the critical temperature, their magnetic propertiesreturn. Iron, for example, has a Curie temperature of about 770° C. Infact, most magnetic materials used in electric machines have Curietemperature far exceeding the operating temperature of the machine. Inmachine 10, however, cylinder 50 and hence the magnetic circuit definedby cylinder 50 is formed of a material (preferably a ferrite) having aCurie temperature above conventional operating temperatures, but below acritical temperature at which damage might be caused to the circuits 58or the remainder of machine 10. For reasons that will become apparent,this Curie temperature may be considered to be the desired shut-downtemperature of machine 10. Preferably, cylinder SO is formed of a “soft”ferrite having a Curie temperature of approximately 200° C. A ferritehaving such property is, for example, a Manganese-Zinc available fromPhillips under material type 3C85, having a Curie temperature of 215° C.of course, other materials may be suitable, and will be easilyidentified by those of ordinary skill in the art. Preferably thematerial will have a Curie temperature between 95° C. and 300° C.depending on the desired shut-down temperature. Of course, some machinedesigns may require lower or higher shut-down temperatures.

In operation then, circuits 58 may be driven by a three-phase powersource, as describe above, causing machine 10 to act as a motor. Insteadof using an alternating current three-phase power source, each ofcircuits 58 a, 58 b and 58 c may be driven by a square wave source, witheach square way source out of phase with another square wave source by120°. As will be appreciated, this has the same effect of using apoly-phase AC source, driving rotor assembly 14.

More significantly, however, machine 10 may be operated as a generatorby driving shaft 40 using a rotational source of mechanical power. Forexample, shaft 40 may be interconnected with the power shaft of a gasturbine engine, and driven at very high speeds (potentially in excess of100,000 rpm). As will be appreciated, rotating rotor assembly 14, andmore particularly magnetic shell 18 will generate a rotating magneticfield about the central axis of rotor assembly 14. This, in turn,establishes an alternating magnetic flux in the magnetic circuit definedby cylinder 50. This flux, in turn, induces an electric current in thewindings defined by circuits 58 a, 58 b and 58 c. As will beappreciated, the current so generated will be three-phase current,having a frequency proportional to the speed of rotation of rotorassembly 14, with current through circuits 58 a, 58 b and 58 c being outof phase with each other by 120°. If desired, this current may berectified using a conventional rectification circuit (also not shown).

Now, in the event machine 10 is subject to an internal fault, such asfor example, caused by a short across conductors 54 or 56, current inthe conductors will increase, resulting in increased heat in theconductors. Moreover, as conductors 54, and 56 are preferably inphysical contact with, and thermally coupled to cylinder 50, increase intemperature of conductors 54 or 56 will be transferred to cylinder 50.As the temperature of cylinder 50 approaches the Curie temperature ofthe material forming cylinder 50, cylinder 50 loses its magneticproperties, thereby severely limiting the flux through cylinder 50 andthe current induced in the windings formed by circuits 58, andeffectively shutting down machine 10 acting as a generator. Clearly, asthe current is reduced, the temperature of the conductors is reduceduntil the temperature of cylinder 50 again drops below the Curietemperature of the material and its magnetic properties return. As willbe apparent, in steady state and in the presence of a fault, machine 10will operate with cylinder 50 at or near the selected shut-down or Curietemperature Clearly, for a properly chosen Curie temperature, cylinder50 acts as temperature activated fuse, limiting the operatingtemperature of machine 10, and thereby any damage to its components.

Additionally, the use of ferrite material in the formation of statorassembly 12 advantageously reduces Hysteresis and Eddy current losseswithin stator assembly 12. This becomes particularly beneficial at highspeeds.

In yet another embodiment, rotor assembly 14 may include a materialhaving the desired shut-down Curie temperature. Preferably, a ferritematerial is placed radially outward of magnets forming part of rotorassembly 14, effectively as part of the magnetic circuit formed couplingthe flux from rotor assembly 14 to stator assembly 12. Cylinder 50 maybe formed of a material having a much higher Curie temperature. Theferrite material on rotor assembly 14 may then be thermally coupled tothe conductors forming circuits 58. These conductors, could for example,be coupled to rotor assembly 14 by radiation or convection. In the eventthat the temperature of these conductors increases, the increase intemperature is conducted to the ferrite portion of the rotor assembly14, thereby causing the ferrite material to lose its magnetic propertiesnear the Curie temperature. This results in a portion of the magneticcircuit about the magnets of rotor assembly 14 having a very lowpermeability, thereby reducing the magnetic flux emanating with rotorassembly and coupled to cylinder 50; the resulting flux in cylinder 50;and the resulting current in circuits 58. Again, at steady state thissecond embodiment will operate with the temperature of the windings androtor at or near the selected shut-down or Curie temperature.

In a further embodiment, a cylinder 50′ illustrated in FIG. 8 may formpart of a machine that is otherwise identical to machine 10, may beformed of more than one material. A portion 62 of the cylinder 50, ofcylinder 50 is preferably formed of ferrite material having the desiredshut-down Curie temperature, and the remaining portion 64 of thecylinder formed of a material having a different Curie temperature. Forexample the toothed portion (ie. the lengthwise extending teeth orridges) of cylinder 50′ may be formed of laminated iron, while theremainder may be formed of Manganese-Zinc having a Curie temperature ofabout 200° C. Individual iron teeth or ridges may be epoxied to aManganese-Zinc portion. Above the Curie temperature, the resultingmagnetic circuit would have a very high reluctance, severely limitingthe magnetic flux guided about rotor assembly 14, and therefore thecurrent through windings about the cylinder 50′, again causing cylinder50′ to operate at or near the chosen Curie temperature. Of course, otherconfigurations of cylinder 50′ having other portions formed of amagnetic material having the desired Curie temperature will be readilyapparent to those of ordinary skill in the art.

Clearly, the above embodiments may be modified in many ways while stillembodying the invention. For example, the shape of cylinder 50 could bemodified—a toroid or other shape could take its place; the arrangementsof conductors and windings could be changed in any number of known ways;the permanent magnet of rotor assembly 14 can be formed in numerousways; and the size of the machine can be scaled (increased or decreased)as required; other magnetic materials having suitable Curie temperaturemay be used. Thus it is apparent that the described invention may beembodied in many ways. As further examples, the invention could beembodied in a salient pole DC machine; or in a synchronous machine.

The present invention is particularly well suited, among other things,to prevent overheating problems of an internally short circuitedpermanent magnet arrangement that is driven continuously. For example,as depicted in FIG. 9, in the case of an internal fault in a machine 70driven by a shaft 71 in gas turbine engine 72. The invention alsopermits a certain level of control to be attained over an alternatorwhich is driven at variable speeds (i.e. driven by an operatingpropulsive aircraft gas turbine).

The above described embodiments, are intended to be illustrative onlyand in no way limiting. The described embodiments of carrying out theinvention, are susceptible to many modifications of form, size,arrangement of parts, and details of operation. The invention, rather,is intended to encompass all such modification within its scope, asdefined by the claims.

1-10. (canceled)
 11. A method of thermally protecting an electricgenerator to prevent overheating, said generator comprising a statormounted about a rotor, and at least one winding about said stator, saidstator at least partially defining a magnetic circuit guiding magneticflux emanating from said rotor, said method comprising: forming at leasta portion of said magnetic circuit from magnetic material having a Curietemperature below a temperature at which said generator is damaged;thermally coupling said winding to said portion of said magneticcircuit, so that said magnetic circuit limits flow of said magnetic fluxabout said magnetic circuit above said Curie temperature, limitingoperating temperature of said windings, and preventing overheating ofsaid generator during operation.
 12. The method of claim 11, furthercomprising choosing said portion of said magnetic material-to have aCurie temperature between 95 and 300 degrees Celsius.
 13. The method ofclaim 12, wherein said portion of said magnetic circuit is formed from aManganese-Zinc ferrite material.
 14. The method of claim 11, furthercomprising providing a thermally conductive compound between said statorand said windings to thermally couples said winding to said stator. 15.A method of providing a thermally limited source of electrical powerwithin an aircraft, said method comprising: providing an electricgenerator comprising a stator mounted about a rotor, and at least onewinding about said stator, said stator at least partially defining amagnetic circuit guiding magnetic flux emanating from said rotor,wherein said magnetic circuit comprises a portion formed from magneticmaterial having a Curie temperature below a temperature at which saidgenerator is damaged; and said stator is thermally coupled to saidwinding, so that said magnetic circuit is thermally coupled to saidwinding and thereby limits flow of said magnetic flux about saidmagnetic circuit above said Curie temperature, limiting operatingtemperature of said windings, and preventing overheating of saidgenerator during operation; driving said rotor using an engine of saidaircraft to generate electrical power from said generator.
 16. Themethod of claim 15, further comprising heating said windings above saidCurie temperature in the presence of a short circuit, thereby heatingsaid portion and limiting flow of said magnetic flux about said magneticcircuit above said Curie temperature, thereby limiting operatingtemperature of said windings, and preventing overheating of saidgenerator during operation.
 17. The method of claim 16, wherein saidheating comprising heating said portion above 95 degrees Celsius tolimit flow of magnetic flux about said magnetic circuit.
 18. The methodof claim 17, wherein said heating comprises heating said portion below300 degrees Celsius to limit flow of magnetic flux about said magneticcircuit.
 19. A method of preventing overheating in an electric generatorin the presence of an internal fault, said generator having a stator, arotor and at least one winding about said stator, said stator at leastpartially defining a magnetic circuit guiding magnetic flux emanatingfrom said rotor, said method comprising: (a) determining a desiredshut-down temperature for said generator which is below a temperature atwhich said generator is thermally damaged; and (b) selecting a generatorhaving at least a portion of said magnetic circuit composed of amagnetic material having a Curie temperature substantially equal to saiddesired shut-down temperature, said winding being thermally coupled tosaid portion of said magnetic circuit, so that in the presence of aninternal fault causing an operating temperature of said windings toincrease, said portion of said magnetic circuit is heated to said Curietemperature, thereby limiting said magnetic flux about said magneticcircuit and current induced in said windings, and thereby preventingoverheating of said generator in the presence of said internal fault.20. The method of claim 19, wherein said shut-down temperature is below300 degrees Celsius.
 21. The method of claim 19, wherein said portion isformed on said stator.
 22. A method of preventing overheating in anelectric generator, said generator having a stator, a rotor and at leastone winding about said stator, said stator at least partially defining amagnetic circuit guiding magnetic flux emanating from said rotor, saidmethod comprising: (a) determining a desired shut-down temperature forsaid generator which is below a temperature at which said generator isdamaged; and (b) selecting a generator having at least a portion of saidmagnetic circuit composed of a magnetic material having a Curietemperature not greater than said desired shut-down temperature, saidwinding being thermally coupled to said portion of said magneticcircuit, so that an increase in operating temperature of said windingscausing said portion of said magnetic circuit to increase to said Curietemperature thereby limits said magnetic flux about said magneticcircuit and current induced in said windings, thereby substantiallyshutting down an electricity generating function of said generator untilsaid portion of said magnetic circuit cools below said Curietemperature.
 23. The method of claim 22, wherein said shut-downtemperature is below 300 degrees Celsius.
 24. The method of claim 22,wherein said portion is formed on said rotor.
 25. A method of providinga thermally limited source of electrical power within an aircraft, saidmethod comprising: (a) determining a desired shut-down temperature for agenerator, (b) selecting a generator having a stator, a rotor and atleast one winding about said stator, said stator at least partiallydefining a magnetic circuit guiding magnetic flux emanating from saidrotor, at least a portion of said magnetic circuit composed of amagnetic material having a Curie temperature not greater than saiddesired shut-down temperature, said winding being thermally coupled tosaid portion of said magnetic circuit; (c) in the presence of anunintended short circuit in said windings, permitting an increase in anoperating temperature of said windings to thereby cause said portion ofsaid magnetic circuit to increase in temperature; and (d) permittingsaid portion of said magnetic circuit to increase in temperature to saidCurie temperature thereof to thereby limit said magnetic flux about saidmagnetic circuit and current induced in said windings, thereby shuttingdown an electricity generating function of said generator.